Question 1164790
in 1 month, arch does 1/4 of it
in 1 month, arch + father do 1/2.4 of it
(1/4)+(1/x)=1/(2.4)
multiply by 9.6x
2.4x+9.6=4x
1.6x=9.6
x=6 months. ANSWER
every month, they do (1/4)+(1/6) or 5/12 of the area
in 2.4 months, they do all of it.